1. Field of the Invention
The present invention relates to a programmable controller used for process control of a plant, and in particular to a microprogram load unit of a programmable controller which can be suitably used for high speed, highly functional control.
2. Description of Related Art
Programmable controllers were used for effecting simple sequence control at the beginning. In recent years, however, programmable controllers have been highly improved in speed, function and reliability. With the expansion in application field, therefore, improvement in function of an existing controller, minimization in its down time, and installation of a controller at the field have been demanded.
In a typical conventional method, the microprogram is stored in a microprogram read only memory without using loading. In this method, however, it is impossible to load a microprogram containing a new function, enhancement in controller function being obviated.
In a method adopted to avoid the above described drawback, a program transfer unit transfers a microprogram from a nonvolatile memory comprising a relatively large-capacity mechanical part, such as a flexible disk, to a readable/writable microprogram memory. This method is relatively simple. Because of use of the mechanical part in the nonvolatile memory, however, it is necessary to maintain a good environment by using air conditioning. Therefore, application of the method has been limited to units installed in places with good surroundings such as an office computer. One of such units is a microprogram load unit proposed in Japanese Patent Unexamined Publication No. 146547/79. In this unit, the transfer of the microprogram to the control unit at the time of power turn on or initialization is effected from the disk unit without being passed through the floppy disk unlike the prior art. Since this unit uses a magnetic disk unit including mechanical parts, it must be installed in a place with good surroundings. Further, due to use of the mechanical parts, the program readout speed is slow, resulting in drawbacks that it takes a long time for loading and the failure time caused by power down is prolonged.
Futher, although the microprogram can be written into or read out of the memory contained in the controller at high speed, a large amount of power dissipation is required to attain the high speed. And, a battery backup is not provided for use in case of a power failure.